Method and apparatus for scalably encoding and decoding video signal

ABSTRACT

In one embodiment, the method includes predicting at least a portion of a current image in a current layer based on at least a residual coded portion of a base image in a base layer, a reference image, shift information for samples in the predicted current image, and offset information indicating a position offset between at least one boundary pixel of the reference image and at least one boundary pixel of the current image. The residual coded portion represents difference pixel data.

DOMESTIC PRIORITY INFORMATION

This application is a continuation-in-part application of applicationSer. Nos. 11/392,634, 11/401,318, 11/401,317, 11/392,674 and 11/392,673filed Mar. 30, 2006, Apr. 11, 2006, Apr. 11, 2006, Mar. 30, 2006 andMar. 30, 2006, respectively, the entire contents of each of which arehereby incorporated by reference.

This application also claims priority under 35 U.S.C. §119 on U.S.Provisional Application No. 60/667,115, filed on Apr. 1, 2005; U.S.Provisional Application No. 60/670,246, filed on Apr. 12, 2005; U.S.Provisional Application No. 60/670,241, filed on Apr. 12, 2005; and U.S.Provisional Application No. 60/670,676 filed Apr. 13, 2005; the entirecontents of each of which are hereby incorporated by reference.

FOREIGN PRIORITY INFORMATION

This application claims priority under 35 U.S.C. §119 on Korean PatentApplication Nos. 10-2005-0084744, 10-2005-0066622, 10-2005-0084729 and10-2005-0084742 filed on Sep. 12, 2005, Jul. 22, 2005, Sep. 12, 2005 andSep. 12, 2005, respectively, the entire contents of each of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to scalable encoding and decoding of avideo signal.

2. Description of the Related Art

It is difficult to allocate high bandwidth, require for TV signals, todigital video signals wirelessly transmitted and received by mobilephones and notebook computers. It is expected that similar difficultieswill occur with mobile TVs and handheld PCs, which will come intowidespread use in the future. Thus, video compression standards for usewith mobile devices should have high video signal compressionefficiencies.

Such mobile devices have a variety of processing and presentationcapabilities so that a variety of compressed video data forms should beprepared. This means that a variety of different quality video data withdifferent combinations of a number of variables such as the number offrames transmitted per second, resolution, and the number of bits perpixel should be provided based on a single video source. This imposes agreat burden on content providers.

Because of the above, content providers prepare high-bitrate compressedvideo data for each source video and perform, when receiving a requestfrom a mobile device, a process of decoding compressed video andencoding it back into video data suited to the video processingcapabilities of the mobile device. However, this method entails atranscoding procedure including decoding, scaling, and encodingprocesses, which causes some time delay in providing the requested datato the mobile device. The transcoding procedure also requires complexhardware and algorithms to cope with the wide variety of target encodingformats.

The Scalable Video Codec (SVC) has been developed in an attempt toovercome these problems. This scheme encodes video into a sequence ofpictures with the highest image quality while ensuring that part of theencoded picture (frame) sequence (specifically, a partial sequence offrames intermittently selected from the total sequence of frames) can bedecoded to produce a certain level of image quality.

Motion Compensated Temporal Filtering (MCTF) is an encoding scheme thathas been suggested for use in the Scalable Video Codec. The MCTF schemehas a high compression efficiency (i.e., a high coding efficiency) forreducing the number of bits transmitted per second. The MCTF scheme islikely to be applied to transmission environments such as a mobilecommunication environment where bandwidth is limited.

Although it is ensured that part of a sequence of pictures encoded inthe scalable MCTF coding scheme can be received and processed to videowith a certain level of image quality as described above, there is stilla problem in that the image quality is significantly reduced if thebitrate is lowered. One solution to this problem is to provide anauxiliary picture sequence for low bitrates, for example, a sequence ofpictures that have a small screen size and/or a low frame rate. Oneexample is to encode and transmit not only a main picture sequence of4CIF (Common Intermediate Format) but also an auxiliary picture sequenceof CIF and an auxiliary picture sequence of QCIF (Quarter CIF) todecoders. Each sequence is referred to as a layer, and the higher of twogiven layers is referred to as an enhanced layer and the lower isreferred to as a base layer.

More often, the auxiliary picture sequence is referred to as a baselayer (BL), and the main picture sequence is referred to as an enhancedor enhancement layer. Video signals of the base and enhanced layers haveredundancy since the same video content is encoded into two layers withdifferent spatial resolution or different frame rates. To increase thecoding efficiency of the enhanced layer, a video signal of the enhancedlayer may be predicted using motion information and/or textureinformation of the base layer. This prediction method is referred to asinter-layer prediction.

FIG. 11 illustrates examples of an intra BL prediction method and aninter-layer residual prediction method, which are inter-layer predictionmethods for encoding the enhanced layer using the base layer.

The intra BL prediction method uses a texture (or image data) of thebase layer. Specifically, the intra BL prediction method producespredictive data of a macroblock of the enhanced layer using acorresponding block of the base layer encoded in an intra mode. The term“corresponding block” refers to a block which is located in a baselayer-frame temporally coincident with a frame including the macroblockand which would have an area covering the macroblock if the base layerframe were enlarged by the ratio of the screen size of the enhancedlayer to the screen size of the base layer. The intra BL predictionmethod uses the corresponding block of the base layer after enlargingthe corresponding block by the ratio of the screen size of the enhancedlayer to the screen size of the base layer through upsampling.

The inter-layer residual prediction method is similar to the intra BLprediction method except that it uses a corresponding block of the baselayer encoded so as to contain residual data, which is data of an imagedifference, rather than a corresponding block of the base layercontaining image data. The inter-layer residual prediction methodproduces predictive data of a macroblock of the enhanced layer encodedso as to contain residual data, which is data of an image difference,using a corresponding block of the base layer encoded so as to containresidual data. Similar to the intra BL prediction method, theinter-layer residual prediction method uses the corresponding block ofthe base layer containing residual data after enlarging thecorresponding block by the ratio of the screen size of the enhancedlayer to the screen size of the base layer through upsampling.

A base layer with lower resolution for use in the inter-layer predictionmethod is produced by downsampling a video source. Correspondingpictures (frames or blocks) in enhanced and base layers produced fromthe same video source may be out of phase since a variety of differentdownsampling techniques and downsampling ratios (i.e., horizontal and/orvertical size reduction ratios) may be employed.

FIG. 12 illustrates a phase relationship between enhanced and baselayers. A base layer may be produced (i) by sampling a video source atlower spatial resolution separately from an enhanced layer or (ii) bydownsampling an enhanced layer with higher spatial resolution. In theexample of FIG. 12, the downsampling ratio between the enhanced and baselayers is 2/3.

A video signal is managed as separate components, namely, a lumacomponent and two chroma components. The luma component is associatedwith luminance information Y and the two chroma components areassociated with chrominance information Cb and Cr. A ratio of 4:2:0(Y:Cb:Cr) between luma and chroma signals is widely used. Samples of thechroma signal are typically located midway between samples of the lumasignal. When an enhanced layer and/or a base layer are produced directlyfrom a video source, luma and chroma signals of the enhanced layerand/or the base layer are sampled so as to satisfy the 4:2:0 ratio and aposition condition according to the 4:2:0 ratio.

In the above case (i), the enhanced and base layers may be out of phaseas shown in section (a) of FIG. 12 since the enhanced and base layersmay have different sampling positions. In the example of section (a),luma and chroma signals of each of the enhanced and base layers satisfythe 4:2:0 ratio and a position condition according to the 4:2:0 ratio.

In the above case (ii), the base layer is produced by downsampling lumaand chroma signals of the enhanced layer by a specific ratio. If thebase layer is produced such that luma and chroma signals of the baselayer are in phase with luma and chroma signals of the enhanced layer,the luma and chroma signals of the base layer do not satisfy a positioncondition according to the 4:2:0 ratio as illustrated in section (b) ofFIG. 12.

In addition, if the base layer is produced such that luma and chromasignals of the base layer satisfy a position condition according to the4:2:0 ratio, the chroma signal of the base layer is out of phase withthe chroma signal of the enhanced layer as illustrated in section (c) ofFIG. 12. In this case, if the chroma signal of the base layer isupsampled by a specific ratio according to the inter-layer predictionmethod, the upsampled chroma signal of the base layer is out of phasewith the chroma signal of the enhanced layer.

Also in case (ii), the enhanced and base layers may be out of phase asillustrated in section (a).

That is, the phase of the base layer may be changed in the downsamplingprocedure for producing the base layer and in the upsampling procedureof the inter-layer prediction method, so that the base layer is out ofphase with the enhanced layer, thereby reducing coding efficiency.

Also, video frames in sequences of different layers may have differentaspect ratios. For example, video frames of the higher sequence (i.e.,the enhanced layer may have a wide aspect ratio of 16:9, whereas videoframes of the lower sequence (i.e., the base layer) may have a narrowaspect ratio of 4:3. In this case, there maybe a need to determine whichpart of a base layer picture is to be used for an enhanced layer pictureor for which part of the enhanced layer picture the base layer pictureis to be used when performing prediction of the enhanced layer picture.

SUMMARY OF THE INVENTION

The present invention relates to a method for decoding a video signal.In one embodiment, the method includes predicting at least a portion ofa current image in a current layer based on at least a residual codedportion of a base image in a base layer, a reference image, shiftinformation for samples in the predicted current image, and offsetinformation indicating a position offset between at least one boundarypixel of the reference image and at least one boundary pixel of thecurrent image. The residual coded portion represents difference pixeldata.

In one embodiment, the reference image is at least an up-sampledresidual coded portion of the base image. For example, in oneembodiment, the method may include upsampling at least the residualcoded portion of the base image based on the shift information to obtainthe reference image. The shift information may be phase shiftinformation.

In one embodiment, the offset information may include left offsetinformation indicating a position offset between at least one left sidepixel of the reference image and at least one left side pixel of thecurrent image, top offset information indicating a position offsetbetween at least one top side pixel of the reference image and at leastone top side pixel of the current image, right offset informationindicating a right position offset between at least one right side pixelof the reference image and at least one right side pixel of the currentimage, and/or bottom offset information indicating a bottom positionoffset between at least one bottom side pixel of the reference image andat least one bottom side pixel of the current image.

In a further embodiment, the predicting step predicts the portion of thecurrent image based on at least part of the reference image, the offsetinformation and dimension information. The dimension informationindicates at least one dimension of the current image. For example, thedimension information may include width information indicating a widthof the current image, and/or height information indicating a height ofthe current image.

The present invention further relates to methods of encoding a videosignal, an apparatus for decoding a video signal, and/or an apparatusfor encoding a video signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 a is a block diagram of an embodiment of a video signal encodingapparatus to which a scalable video signal coding method according tothe present invention is applied;

FIG. 1 b is a block diagram of another embodiment of a video signalencoding apparatus to which a scalable video signal coding methodaccording to the present invention is applied;

FIG. 2 is a block diagram of part of an EL encoder shown in FIGS. 1 aand 1 b responsible for carrying out image estimation/prediction andupdate operations;

FIGS. 3 a and 3 b illustrate the relationship between enhanced layerframes and base layer frames which can be used as reference frames forconverting an enhanced layer frame to an H frame having a predictiveimage;

FIG. 4 illustrates how part of a base layer picture is selected andenlarged to be used for a prediction operation of an enhanced layerpicture according to an embodiment of the present invention;

FIGS. 5 a and 5 b illustrate embodiments of the structure of informationregarding a positional relationship of a base layer picture to anenhanced layer picture, which is transmitted to the decoder, accordingto the present invention;

FIG. 6 illustrates how an area including a base layer picture isenlarged to be used for a prediction operation of an enhanced layerpicture according to another embodiment of the present invention;

FIG. 7 illustrates how a base layer picture is enlarged to a larger areathan an enhanced layer picture so as to be used for a predictionoperation of the enhanced layer picture according to yet anotherembodiment of the present invention;

FIG. 8 is a block diagram of an apparatus for decoding a data streamencoded by the apparatus of FIG. 1;

FIG. 9 is a block diagram of part of an EL decoder shown in FIG. 8responsible for carrying out inverse prediction and update operations;

FIG. 10 illustrates a method for upsampling a base layer for use indecoding an enhanced layer, encoded according to an inter-layerprediction method, taking into account a phase shift in the base layerand/or the enhanced layer, according to an embodiment of the presentinvention;

FIG. 11 illustrates an example of an inter-layer prediction method forencoding an enhanced layer using a base layer according to conventionalart and

FIG. 12 illustrates examples of phase relationships between enhanced andbase layers according to conventional art.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Example embodiments of the present invention will now be described indetail with reference to the accompanying drawings.

FIG. 1 a is a block diagram of a video signal encoding apparatus towhich a scalable video signal coding method according to the presentinvention is applied. Although the apparatus of FIG. 1 a is implementedto code an input video signal in two layers, principles of the presentinvention described below can also be applied when a video signal iscoded in three or more layers. The present invention can also be appliedto any scalable video coding scheme, without being limited to an MCTFscheme which is described below as an example.

The video signal encoding apparatus shown in FIG. 1 a comprises anenhanced layer (EL) encoder 100 to which the present invention isapplied, a texture coding unit 110, a motion coding unit 120, a baselayer encoder 150, and a muxer (or multiplexer) 130. The EL encoder 100is an enhanced layer encoder which encodes an input video signal on aper macroblock basis according to a specified encoding scheme (e.g., anMCTF scheme) and generates suitable management information. The texturecoding unit 110 converts information of encoded macroblocks into acompressed bitstream. The motion coding unit 120 codes motion vectors ofimage blocks obtained by the EL encoder 100 into a compressed bitstreamaccording to a specified scheme. The base layer encoder 150 encodes aninput video signal according to a specified scheme, for example,according to the MPEG-1, 2 or 4 standard or the H.261, H.263 or H.264standard, and produces a small-screen picture sequence, for example, asequence of pictures scaled down to 25% of their original size. Themuxer 130 encapsulates the output data of the texture coding unit 110,the small-screen picture sequence output from the base layer encoder150, and the output vector data of the motion coding unit 120 into adesired format. The muxer 130 then multiplexes and outputs theencapsulated data into a desired transmission format.

The base layer encoder 150 can provide a low-bitrate data stream notonly by encoding an input video signal into a sequence of pictureshaving a smaller screen size than pictures of the enhanced layer, butalso by encoding an input video signal into a sequence of pictureshaving the same seen size as pictures of the enhanced layer at a lowerframe rate than the enhanced layer. In the embodiments of the presentinvention described below, the base layer is encoded into a small-screenpicture sequence, and the small-screen picture sequence is referred toas a base layer sequence and the frame sequence output from the ELencoder 100 is referred to as an enhanced layer sequence.

FIG. 1 b illustrates a block diagram of another embodiment video signalencoding apparatus to which a scalable video signal coding methodaccording to the present invention is applied. This embodiment is thesame as the embodiment of FIG. 1 a except for the addition of thedownsampling unit 140. The downsampling unit 140 produces an enhancedlayer signal directly from an input video signal or by downsampling theinput video signal, and produces a base layer signal by downsampling theinput video signal or the enhanced layer signal according to a specificscheme. The specific scheme will depend on the applications or devicesreceiving each layer, and therefore, is a matter of design choice. Here,the EL encoder 100 encodes the enhanced layer signal generated by thedownsampling unit 140 on a per macroblock basis in a scalable fashionaccording to a specified encoding scheme (for example, an MCTF scheme),and generates suitable management information. Also, the BL encoder 150encodes the base layer signal generated by the downsampling unit 140according to a specified scheme, for example, according to the MPEG-1, 2or 4 standard or the H.261 or H.264 standard, and produces asmall-screen picture sequence, for example, a sequence of picturesscaled down to 25% of their original size if needed.

In each embodiment, the EL encoder 100 performs motion estimation andprediction operations on each target macroblock in a video frame. The ELencoder 100 also performs an update operation for each target macroblockby adding an image difference of the target macroblock from acorresponding macroblock in a neighbor frame to the correspondingmacroblock in the neighbor frame. FIG. 2 illustrates some elements ofthe EL encoder 100 for carrying out these operations.

The elements of the EL encoder 100 shown in FIG. 2 include anestimator/predictor 102, an updater 103, and a decoder 105. The decoder105 decodes an encoded stream received from the base layer encoder 150,and enlarges decoded small-screen frames to the size of frames in theenhanced layer using an internal scaler 105 a. The estimator/predictor102 searches for a reference block of each macroblock in a currentframe, which is to be coded into residual data, in adjacent frames priorto or subsequent to the current frame and in frames enlarged by thescaler 105 a. The estimator/predictor 102 then obtains an imagedifference (i.e., a pixel-to-pixel difference) of each macroblock in thecurrent frame from the reference block or from a corresponding block ina temporally coincident frame enlarged by the scaler 105 a, and codesthe image difference into the macroblock. The estimator/predictor 102also obtains a motion vector originating from the macroblock andextending to the reference block. The updater 103 performs an updateoperation for a macroblock in the current frame, whose reference blockhas been found in frames prior to or subsequent to the current frame, bymultiplying the image difference of the macroblock by an appropriateconstant (for example, ½ or ¼) and adding the resulting value to thereference block. The operation carried out by the updater 103 isrefereed to as a ‘U’ operation, and a frame produced by the ‘U’operation is referred to as an ‘L’ frame.

The estimator/predictor 102 and the updater 103 of FIG. 2 may performtheir operations on a plurality of slices, which are produced bydividing a single frame, simultaneously and in parallel instead ofperforming their operations on the video frame. A frame (or slice)having an image difference, which is produced by the estimator/predictor102, is referred to as an ‘H’ frame (or slice). The ‘H’ frame (or slice)contains data having high frequency components of the video signal. Inthe following description of the embodiments, the term ‘picture’ is usedto indicate a slice or a frame, provided that the use of the term istechnically feasible.

The estimator/predictor 102 divides each of the input video frames (or Lframes obtained at the previous level) into macroblocks of a desiredsize. For each divided macroblock the estimator/predictor 102 searchesfor a block whose image is most similar to that of each dividedmacroblock in previous/next neighbor frames of the enhanced layer and/orin base layer frames enlarged by the scaler 105 a. That is, theestimator/predictor 102 searches for a macroblock temporally correlatedwith each divided macroblock. A block having the most similar image to atarget image block has the smallest image difference from the targetimage block. The image difference of two image blocks is defined, forexample, as the sum or average of pixel-to-pixel differences of the twoimage blocks. Of blocks having a threshold image difference or less froma target macroblock in the current frame, a block having the smallestimage difference from the target macroblock is referred to as areference block A picture including the reference block is referred toas a reference picture. For each macroblock of the current frame, tworeference blocks (or two reference pictures) may be present in a frame(including a base layer frame) prior to the current frame, in a frame(including a base layer frame) subsequent thereto, or one in a priorframe and one in a subsequent frame.

If the reference block is found, the estimator/predictor 102 calculatesand outputs a motion vector from the current block to the referenceblock The estimator/predictor 102 also calculates and outputs pixelerror values (i.e., pixel difference values) of the current block frompixel values of the reference block which is present in either the priorframe or the subsequent frame, or form average pixel values of the tworeference blocks, which are present in the prior and subsequent frames.The image or pixel difference values are also referred to as residualdata.

If no macroblock having a desired threshold image difference or lessfrom the current macroblock is found in the two neighbor frames(including base layer frames) via the motion estimation operation, theestimator/predictor 102 determines whether or not a frame in the sametime zone as the current frame (hereinafter also referred to as a“temporally coincident frame”) or a frame in a close time zone to thecurrent frame (hereinafter also referred to as a “temporally closeframe”) is present in the base layer sequence. If such a frame ispresent in the base layer sequence, the estimator/predictor 102 obtainsthe image difference (i.e., residual data) of the current macroblockfrom a corresponding macroblock in the temporally coincident or closeframe based on pixel values of the two macroblocks, and does not obtaina motion vector of the current macroblock with respect to thecorresponding macroblock. The close time zone to the current framecorresponds to a time interval including frames that can be regarded ashaving the same image as the current frame. Information of this timeinterval is carried within an encoded stream.

The above operation of the estimator/predictor 102 is referred to as a‘P’ operation. When the estimator/predictor 102 performs the ‘P’operation to produce an H frame by searching for a reference block ofeach macroblock in the current frame and coding each macroblock intoresidual data, the estimator/predictor 102 can selectively use, asreference pictures, enlarged pictures of the base layer received fromthe scaler 105 a, in addition to neighbor L frames of the enhanced layerprior to and subsequent to the current frame, as shown in FIG. 3 a.

In an example embodiment of the present invention, five frames are usedto produce each H frame. FIG. 3 b shows five frames that can be used toproduce an H frame. As shown, a current L frame 400L has L frames 401prior to and L frames 402 subsequent to the current L frame 400L. Thecurrent L frame 400L also has a base layer frame 405 in the same timezone. One or two frames from among the L frames 401 and 402 in the sameMCTF level as a current L frame 400L, the frame 405 of the base layer inthe same time zone as the L frame 400L, and base layer frames 403 and404 prior to and subsequent to the frame 405 are used as referencepictures to produce an H frame 400H from the current L frame 400L. Aswill be appreciated from the above discussion, there are variousreference block selection modes. To inform the decoder of which mode isemployed, the EL encoder 100 transmits ‘reference block selection mode’information to the texture coding unit 110 after inserting/writing itinto a field at a specified position of a header area of a correspondingmacroblock.

When a picture of the base layer is selected as a reference picture forprediction of a picture of the enhanced layer in the reference pictureselection method as shown in FIG. 3 b, all or part of the base layerpicture can be used for prediction of the enhanced layer picture. Forexample, as shown in FIG. 4, when a base layer picture has an aspectratio of 4:3, an actual image portion 502 of the base layer picture hasan aspect ratio of 16:9, and an enhanced layer picture 500 has an aspectratio of 16:9, upper and lower horizontal portions 501 a and 501 b ofthe base layer picture contain invalid data. In this case, only theimage portion 502 of the base layer picture is used for prediction ofthe enhanced layer picture 500. To accomplish this, the scaler 105 aselects (or crops) the image portion 502 of the base layer picture(S41), up-samples the selected image portion 502 to enlarge it to thesize of the enhanced layer picture 500 (S42), and provides the enlargedimage portion to the estimator/predictor 102.

The EL encoder 100 incorporates position information of the selectedportion of the base layer picture into a header of the current picturecoded into residual data. The EL encoder 100 also sets and inserts aflag “flag_base_layer_cropping”, which indicates that part of the baselayer picture has been selected and used, in the picture header at anappropriate position so that the flag is delivered to the decoder. Theposition information is not transmitted when the flag“flag_base_layer_cropping” is reset.

FIGS. 5 a and 5 b illustrate embodiments of the structure of informationregarding a selected portion 512 of a base layer picture. In theembodiment of FIG. 5 a, the selected portion 512 of the base layerpicture is specified by offsets (left_offset, right_offset, top_offset,and bottom_offset) from the left, right, top and bottom boundaries ofthe base layer picture. The left offset indicates a position offsetbetween left side pixels (or, for example, at least one pixel) in thebase layer image and left side pixels in the selected portion 512. Thetop offset indicates a position offset between top side pixels (or, forexample, at least one pixel) in the base layer image and top side pixelsin the selected portion 512. The right offset indicates a positionoffset between right side pixels (or, for example, at least one pixel)in the base layer image and right side pixels in the selected portion512. The bottom side offset indicates a position offset between bottomside pixels (or, for example, at least one pixel) in the base layerimage and bottom side pixels in the selected portion 512. In theembodiment of FIG. 5 b, the selected portion 512 of the base layerpicture is specified by offsets (left_offset and top_offset) from theleft and top boundaries of the base layer picture and by the width andheight (crop_width and crop_height) of the selected portion 512. Variousother specifying methods are also possible.

The offsets in the information of the selected portion shown in FIGS. 5a and 5 b may have negative values. For example, as shown in FIG. 6,when a base layer picture has an aspect ratio of 4:3, an enhanced layerpicture 600 has an aspect ratio of 16:9, and an actual image portion ofthe picture has an aspect ratio of 4:3, the left and right offset values(left_offset and right_offset) have negative values −d_(L) and −d_(R).Portions 601 a and 601 b extended from the base layer picture arespecified by the negative values −d_(L) and −d_(R). The extendedportions 601 a and 601 b are padded with offscreen data, and a picture610 including the extended portions 601 a and 601 b is upsampled to havethe same size as that of the enhanced layer picture 600. Accordingly,data of an area 611 in the enlarged base layer picture, whichcorresponds to an actual image portion of the enhanced layer picture600, can be used for prediction of the actual image portion of theenhanced layer picture 600.

Since the offset fields of the information illustrated in FIGS. 5 a and5 b may have negative values, the same advantages as described above inthe example of FIG. 4 can be achieved by using the information of FIGS.5 a and 5 b as position information of an area overlapping with theenhanced layer picture, which is to be associated with the enlarged baselayer picture, instead of using the information of FIGS. 5 a and 5 b forspecifying the selected area in the base layer picture.

Specifically, with reference to FIG. 7, when a base layer picture 702 isupsampled so that an actual image area 701 of the base layer picture 702is enlarged to the size of an enhanced layer picture 700, the enlarged(e.g., up-sampled) picture corresponds to an area larger than theenhanced layer picture 700. In this example, top and bottom offsetstop_offset and bottom_offset are included in the position information ofan area overlapping with the enhanced layer picture 700. These offsetscorrespond to the enlarged base layer picture, and are assigned negativevalues −d_(T) and −d_(B) so that only an actual image area of theenlarged base layer picture is used for prediction of the enhanced layerpicture 700. In the example of FIG. 7, left and right offsets of theposition information of the area corresponding to the enlarged baselayer picture are zero. However, it will be understood that the left andright offsets may be non-zero, and also correspond to the enlarged baselayer picture. It will also be appreciated that a portion of the imagein the enlarged base layer picture may not be used in determining theenhanced layer picture. Similarly, when the offset informationcorresponds directly to the base layer picture, as opposed to theup-sample base layer picture, a portion of the image in the base layerpicture may not be used in determining the enhanced layer picture.

Furthermore, in the embodiment of FIG. 7, the left offset indicates aposition offset between left side pixels (or, for example, at least onepixel) in the up-sampled base layer image and left side pixels in theenhanced layer image. The top offset indicates a position offset betweentop side pixels (or, for example, at least one pixel) in the up-sampledbase layer image and top side pixels in the enhanced layer image. Theright offset indicates a position offset between right side pixels (or,for example, at least one pixel) in the up-sampled base layer image andright side pixels in the enhanced layer image. The bottom side offsetindicates a position offset between bottom side pixels (or, for example,at least one pixel) in the up-sampled base layer image and bottom sidepixels in the enhanced layer image.

As described above, the information of FIGS. 5 a and 5 b can be used asinformation for selection of a portion of a base layer picture, which isto be used for prediction of an enhanced layer picture, or can be usedas position information of an area overlapping with an enhanced layerpicture, which is to be associated with a base layer picture for use inprediction of the enhanced layer picture.

Information of the size and aspect ratio of the base layer picture, modeinformation of an actual image of the base layer picture, etc., can bedetermined by decoding, for example, from a sequence header of theencoded base layer stream. Namely, the information may be recorded inthe sequence header of the encoded base layer stream. Accordingly, theposition of an area overlapping with the enhanced layer picture, whichcorresponds to the base layer picture or the selected area in the baselayer picture described above, are determined based on position oroffset information, and all or part of the base layer picture is used tosuit this determination.

Returning to FIGS. 1 a, 1 b and 2, the EL encoder 100 generates asequence of H frames and a sequence of L frames, respectively, byperforming the ‘P’ and ‘U’ operations described above on acertain-length sequence of pictures, for example, on a group of pictures(GOP). Then, an estimator/predictor and an updater at a nextserially-connected stage (not shown) generates a sequence of H framesand a sequence of L frames by repeating the ‘P’ and ‘U’ operations onthe generated L frame sequence. The ‘P’ and ‘U’ operations are performedan appropriate number of times (for example, until one L frame isproduced per GOP) to produce a final enhanced layer sequence.

The data stream encoded in the method described above is transmitted bywire or wirelessly to a decoding apparatus or is delivered via recordingmedia. The decoding apparatus reconstructs the ordinal video signal inthe enhanced and/or base layer according to the method describe below.

FIG. 8 is a block diagram of an apparatus for decoding a data streamencoded by the apparatus of FIG. 1 a or 1 b. The decoding apparatus ofFIG. 8 includes a demuxer (or demultiplexer) 200, a texture decodingunit 210, a motion decoding unit 220, an EL decoder 230, and a baselayer decoder 240. The demuxer 200 separates a received data stream intoa compressed motion vector stream, a compressed macroblock informationstream, and a base layer stream. The texture decoding unit 210reconstructs the compressed macroblock information stream to itsoriginal uncompressed state. The motion decoding unit 220 reconstructsthe compressed motion vector stream to its original uncompressed state.The EL decoder 230 is an enhanced layer decoder which converts theuncompressed macroblock information stream and the uncompressed motionvector stream back to an original video signal, for example, accordingto an MCTF scheme. The base layer decoder 240 decodes the base layerstream according to a specified scheme, for example, according to theMPEG-4 or H.264 standard.

The EL decoder 230 includes, as an internal element, an inverse filterthat has a structure as shown in FIG. 9 for reconstructing an inputstream to its original frame sequence.

FIG. 9 shows some elements of the inverse filter for reconstructing asequence of H and L frames of MCTF level N to a sequence of L frames oflevel N−1. The elements of the inverse filter of FIG. 9 include aninverse updater 231, an inverse predictor 232, a motion vector decoder235, an arranger 234, and a scaler 230 a. The inverse updater 231subtracts pixel difference values of input H frames from correspondingpixel values of input L frames. The inverse predictor 232 reconstructsinput H frames to frames having original images with reference to the Lframes, from which the image differences of the H frames have beensubtracted, and/or with reference to enlarged pictures output from thescaler 240 a. The motion vector decoder 235 decodes an input motionvector stream into motion vector information of each block and providesthe motion vector information to an inverse predictor (for example, theinverse predictor 232) of each stage. The arranger 234 interleaves theframes completed by the inverse predictor 232 between the L framesoutput from the inverse updater 231, thereby producing a normal videoframe sequence. The scaler 230 a enlarges small-screen pictures of thebase layer to the enhanced layer picture size, for example, according tothe information as shown in FIGS. 5 a and 5 b.

The L frames output from the arranger 234 constitute an L frame sequence601 of level N−1. A next-stage inverse updater and predictor of levelN−1 reconstructs the L frame sequence 601 and an input H frame sequence602 of level N−1 to an L frame sequence. This decoding process isperformed the same number of times as the number of MCTF levels employedin the encoding procedure, thereby reconstructing an original videoframe sequence. With reference to ‘reference_selection_code’ informationcarried in a header of each macroblock of an input H frame, the inversepredictor 232 specifies an L frame of the enhanced layer and/or anenlarged frame of the base layer which has been used as a referenceframe to code the macroblock to residual data. The inverse predictor 232determines a reference block in the specified frame based on a motionvector provided from the motion vector decoder 235, and then adds pixelvalues of the reference block (or average pixel values of twomacroblocks used as reference blocks of the macroblock) to pixeldifference values of the macroblock of the H frame; therebyreconstructing the original image of the macroblock of the H frame.

When a base layer picture has been used as a reference frame of acurrent H frame, the scaler 230 a selects and enlarges an area in thebase layer picture (in the example of FIG. 4) or enlarges a larger areathan the base layer picture (in the example of FIG. 6) based onpositional relationship information as shown in FIGS. 5 a and 5 bincluded in a header analyzed by the EL decoder 230 so that the enlargedarea of the base layer picture is used for reconstructing macroblockscontaining residual data in the current H frame to original image blocksas described above. The positional relationship information is extractedfrom the header and is then referred to when information indicatingwhether or not the positional relationship information is included(specifically, the flag “flag_base_layer_cropping” in the example ofFIGS. 5 a and 5 b) indicates that the positional relationshipinformation is included.

In the case where the information of FIGS. 5 a and 5 b has been used asinformation indicating the position of an area overlapping with anenhanced layer picture, to use in prediction of the enhanced layerpicture, the inverse predictor 232 uses an enlarged one of the baselayer picture received from the scaler 230 a for prediction of theenhanced layer picture by associating the entirety of the enlarged baselayer picture with all or part of the current H frame or with a largerarea than the current H frame according to the values (positive ornegative) of the offset information. In the case of FIG. 7 where theenlarged base layer picture is associated with a larger area than thecurrent H frame, the predictor 232 uses only an area in the enlargedbase layer picture, which corresponds to the H frame, for reconstructingmacroblocks in the current H frame to their original images. In thisexample, the offset information included negative values.

For one H frame, the MCTF decoding is performed in specified units, forexample, in units of slices in a parallel fashion, so that themacroblocks in the frame have their original images reconstructed andthe reconstructed macroblocks are then combined to constitute a completevideo frame.

The above decoding method reconstructs an MCTF-encoded data stream to acomplete video frame sequence. The decoding apparatus decodes andoutputs a base layer sequence or decodes and outputs an enhanced layersequence using the base layer depending on its processing andpresentation capabilities.

The decoding apparatus described above may be incorporated into a mobilecommunication terminal, a media player, or the like.

Returning to FIG. 1 b, in addition to the discussion above, oralternative thereto, the downsampling unit 140 not only transmits theenhanced and base layer signals to the EL and BL encoders 100 and 150,but also transmits sampling-related information of the two layers to theEL and BL encoders 100 and 150. The sampling-related information of thetwo layers may include spatial resolution (or screen sizes), framerates, the ratios between luma and chroma signals of the two layers, thepositions of chroma signals of the two layers, and information regardinga phase shift between luma and chroma signals of the two layers based onthe respective positions of the luma and chroma signals of the twolayers.

The phase shift can be defined as the phase difference between lumasignals of the two layers. Typically, luma and chroma signals of the twolayers are sampled so as to satisfy a position condition according tothe ratio between the luma and chroma signals, and the luma signals ofthe two layers are sampled so as to be in phase with each other.

The phase shift can also be defined as the phase difference betweenchroma signals of the two layers. The phase difference between chromasignals of the two layers can be determined based on the differencebetween positions of the chroma signals of the two layers after thepositions of the luma signals of the two layers are matched to eachother so that the luma signals of the two layers are in phase with eachother.

The phase shift can also be individually defined for each layer, forexample, with reference to a single virtual layer (e.g., an upsampledbase layer) based on the input video signal for generating the enhancedor base layer. Here, the phase difference is between luma and/or chromasamples (i.e., pixels) of the enhanced layer of the base layer and thevirtual layer (e.g., an upsampled base layer).

The EL encoder 100 records the phase shift information transmitted fromthe downsampling unit 140 in a header area of a sequence layer or aslice layer. If the phase shift information has a value other than 0,the EL encoder 100 sets a global shift flag “global_shift_flag”, whichindicates whether or not there is a phase shift between the two layers,to, for example, “1”, and records the value of the phase shift ininformation in fields “global_shift_x” and “global_shift_y”. The“global_shift_x” value represents the horizontal phase shift. The“global_shift_y” value represents the vertical phase shift. Statedanother way, the “global_shift_x” value represents the horizontalposition offset between the samples (i.e., pixels), and the“global_shift_y” represents the vertical position offset between thesamples (i.e., pixels).

On the other hand, if the phase shift information has a value of 0, theEL encoder 100 sets the flag “global_shift_flag” to, for example, “0”,and does not record the values of the phase shift in the informationfields “global_shift_x” and “global_shift_y”.

The EL encoder 100 also records the sampling-related information in theheader area of the sequence layer or the slice layer if needed.

It will be recalled from the discussion of FIG. 2 that the EL encoder100 may also include a BL decoder 105. The BL decoder 105 extractsencoding information such as a macroblock mode from an encoded baselayer stream containing a small-screen sequence received from the BLencoder 150, and decodes the encoded base layer stream to produceframes, each composed of one or more macroblocks. It will also berecalled that the estimator/predictor 102 can also search for areference block of the macroblock in a frame of the base layer accordingto the intra BL prediction method. Specifically, the estimator/predictor102 searches for a corresponding block encoded in an intra mode in aframe of the base layer reconstructed by the BL decoder 105, which istemporally coincident with the frame including the macroblock As will berecalled, the term “corresponding block” refers to, for example, a blockwhich is located in the temporally coincident base layer frame and whichwould have an area covering the macroblock if the base layer frame wereenlarged by the ratio of the screen size of the enhanced layer to thescreen size of the base layer.

The estimator/predictor 102 reconstructs an original image of the foundcorresponding block by decoding the intra-coded pixel values of thecorresponding block, and then upsamples the found corresponding block toenlarge it by the ratio of the screen size of the enhanced layer to thescreen size of the base layer. The estimator/predictor 102 performs thisupsampling taking into account the phase shift information“global_shift_x/y” transmitted from the downsampling unit 140 so thatthe enlarged corresponding block of the base layer is in phase with themacroblock of the enhanced layer.

The estimator/predictor 102 encodes the macroblock with reference to acorresponding area in the corresponding block of the base layer, whichhas been enlarged so as to be in phase with the macroblock. Here, theterm “corresponding area” refers to a partial area in the correspondingblock which is at the same relative position in the frame as themacroblock.

If needed, the estimator/predictor 102 searches for a reference areamore highly correlated with the macroblock in the enlarged correspondingblock of the base layer by performing motion estimation on themacroblock while changing the phase of the corresponding block, andencodes the macroblock using the found reference area.

If the phase of the enlarged corresponding block is further changedwhile the reference area is searched for, the estimator/predictor 102sets a local shift flag “local_shift_flag”, which indicates whether ornot there is a phase shift, different from the global phase shift“global_shift_x/y”, between the macroblock and the correspondingupsampled block, to, for example, “1”. Also, the estimator/predictor 102records the local shift flag in a header area of the macroblock andrecords the local phase shift between the macroblock and thecorresponding block in information fields “local_shift_x” and“local_shift_y”. The local phase shift information may be replacementinformation, and provide the entire phase shift information as areplacement or substitute for the global phase shift information.Alternatively, the local phase shift information may be additiveinformation, wherein the local phase shift information added to thecorresponding global phase shift information provides the entire ortotal phase shift information.

The estimator/predictor 102 further inserts information indicating thatthe macroblock of the enhanced layer has been encoded in an intra BLmode in the header area of the macroblock so as to inform the decoder ofthe same.

The estimator/predictor 102 can also apply the inter-layer residualprediction method to a macroblock to contain residual data, which isdata of an image difference, using a reference block found in otherframes prior to and subsequent to the macroblock Also in this case, theestimator/predictor 102 upsamples a corresponding block of the baselayer encoded so as to contain residual data, which is data of an imagedifference, taking into account the phase shift information“global_shift_x/y” transmitted from the downsampling unit 140 so thatthe base layer is in phase with the enhanced layer. Here, thecorresponding block of the base layer is a block which has been encodedso as to contain residual data, which is data of an image difference.

The estimator/predictor 102 inserts information indicating that themacroblock of the enhanced layer has been encoded according to theinter-layer residual prediction method in the header area of themacroblock so as to inform the decoder of the same.

The estimator/predictor 102 performs the above procedure for allmacroblocks in the frame to complete an H frame which is a predictiveimage of the frame. The estimator/predictor 102 performs the aboveprocedure for all input video frames or all odd ones of the L framesobtained at the previous level to complete H frames which are predictiveimages of the input frames.

As described above, the updater 103 adds an image difference of eachmacroblock in an H frame produced by the estimator/predictor 102 to an Lframe having its reference block, which is an input video frame or aneven one of the L frames obtained at the previous level.

The data stream encoded in the method described above is transmitted bywire or wirelessly to a decoding apparatus or is delivered via recordingmedia. The decoding apparatus reconstructs the original video signalaccording to the method described below.

FIG. 10 illustrates a method for upsampling a base layer for use indecoding an enhanced layer, encoded according to the inter-layerprediction method, taking into account a phase shift in the base layerand/or the enhanced layer, according to an embodiment of the presentinvention.

In order to decode a macroblock of the enhanced layer encoded accordingto the inter-layer prediction method, a block of the base layercorresponding to the macroblock is enlarged by the ratio of the screensize of the enhanced layer to the screen size of the base layer throughupsampling. This upsampling is performed taking into account phase shiftinformation “global_shift_x/y” in the enhanced layer and/or the baselayer, so as to compensate for a global phase shift between themacroblock of the enhanced layer and the enlarged corresponding block ofthe base layer.

If there is a local phase shift “local_shift_x/y”, different from theglobal phase shift “global_shift_x/y”, between the macroblock of theenhanced layer and the corresponding block of the base layer, thecorresponding block is upsampled taking into account the local phaseshift “local_shift_x/y”. For example, the local phase shift informationmay be used instead of the global phase shift information in oneembodiment, or alternatively, in addition to the global phase shiftinformation in another embodiment

Then, an original image of the macroblock of the enhanced layer isreconstructed using the corresponding block which has been enlarged soas to be in phase with the macroblock

Returning to FIGS. 8 and 9, decoding of macroblocks based on theabove-described phase shift information will now be described. Only thedifferences from the description of decoding provided above with respectto FIGS. 8 and 9 will be described in detail for the sake of brevity. Inthis embodiment, the EL decoder 230 uses encoding information of thebase layer and/or a decoded frame or macroblock of the base layer inorder to decode an enhanced layer stream according to the inter-layerprediction method. To accomplish this, the EL decoder 230 reads a globalshift flag “global_shift_flat” and phase shift information“global_shift_x/y” from a sequence header area or a slice header area ofthe enhanced layer to determine whether or not there is a phase shift inthe enhanced layer and/or the base layer and to confirm the phase shift.The EL decoder 230 upsamples the base layer taking into account theconfirmed phase shift so that the base layer to be used for theinter-layer prediction method is in phase with the enhanced layer.

More specifically, with reference to FIG. 9, the inverse predictor 232reconstructs an original image of an intra-coded block in the baselayer, which corresponds to the macroblock in the enhanced layer, andupsamples the reconstructed corresponding block from the base layer toenlarge it by the ratio of the screen size of the enhanced layer to thescreen size of the base layer. The inverse predictor 232 performs thisupsampling taking into account phase shift information“global_shift_x/y” in the enhanced layer and/or the base layer so thatthe enlarged corresponding block of the base layer is in phase with themacroblock of the enhanced layer. Namely, if the “global_shift_flag”indicates a phase shift exists between the base layer and the enhancedlayer (e.g., equals 1), then the inverse predictor 232 phase shifts thecorresponding macroblock from the base layer during upsampling by the“global_shift_x” and “global_shift_y” values. The inverse predictor 232reconstructs an original image of the macroblock by adding pixel valuesof a corresponding area in the enlarged corresponding block of the baselayer, which has been enlarged so as to be in phase with the macroblock,to the difference values of pixels of the macroblock. Here, the term“corresponding area” refers to a partial area in the corresponding blockwhich is at the same relative position in the frame as the macroblock.

If a local shift flag “local_shift_flag” indicates that there is a localphase shift “local_shift_x/y” different from the global phase shift“global_shift_x/y” between the macroblock and the corresponding block,the inverse predictor 232 upsamples the corresponding block taking intoaccount the local phase shift “local_shift_x/y” (as substitute oradditional phase shift information). The local phase shift informationmay be included in the header area of the macroblock.

If information indicating that a macroblock in an H frame has beenencoded in an inter-layer residual mode is included in a header area ofthe macroblock the inverse predictor 232 upsamples a corresponding blockof the base layer encoded so as to contain residual data, taking intoaccount the global phase shift “global_shift_x/y” as discussed above toenlarge the corresponding block so as to be in phase with the macroblockof the enhanced layer. The inverse predictor 232 then reconstructsresidual data of the macroblock using the corresponding block enlargedso as to be in phase with the macroblock.

The inverse predictor 232 searches for a reference block of thereconstructed macroblock containing residual data in an L frame withreference to a motion vector provided from the motion vector decoder233, and reconstructs an original image of the macroblock by addingpixel values of the reference block to difference values of pixels(i.e., residual data) of the macroblock.

All macroblocks in the current H frame are reconstructed to theiroriginal images in the same manner as the above operation, and thereconstructed macroblocks are combined to reconstruct the current Hframe to an L frame. The arranger 234 alternately arranges L framesreconstructed by the inverse predictor 232 and L frames updated by theinverse updater 231, and outputs such arranged L frames to the nextstage.

The above decoding method reconstructs an MCTF-encoded data stream to acomplete video frame sequence. In the case where the prediction andupdate operations have been performed for a group of pictures (GOP) Ntimes in the MCTF encoding procedure described above, a video framesequence with the original image quality is obtained if the inverseupdate and prediction operations are performed N times in the MCTFdecoding procedure. However, a video frame sequence with a lower imagequality and at a lower bitrate may be obtained if the inverse update andprediction operations are performed less than N times. Accordingly, thedecoding apparatus is designed to perform inverse update and predictionoperations to the extent suitable for the performance thereof.

The decoding apparatus described above can be incorporated into a mobilecommunication terminal, a media player, or the like.

As is apparent from the above description, a method and apparatus forencoding/decoding a video signal according to the present invention usespictures of a base layer provided for low-performance decoders, inaddition to pictures of an enhanced layer, when encoding a video signalin a scalable fashion, so that the total amount of coded data isreduced, thereby increasing coding efficiency. In addition, part of abase layer picture, which can be used for a prediction operation of anenhanced layer picture, is specified so that the prediction operationcan be performed normally without performance degradation even when apicture enlarged from the base layer picture cannot be directly used forthe prediction operation of the enhanced layer picture.

As is apparent from the above description, a method for encoding anddecoding a video signal according to the present invention increasescoding efficiency by preventing a phase shift in a base layer and/or anenhanced layer caused in downsampling and upsampling procedures whenencoding/decoding the video signal according to an inter-layerprediction method.

Furthermore, as will be apparent from the descriptions provided above,the encoding and decoding embodiments related to phase shift informationmay be used independently or in conjunction with the encoding anddecoding embodiments related to offset information.

Although the example embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various improvements, modifications, substitutions, andadditions are possible, without departing from the scope and spirit ofthe invention.

1. A method for decoding a video signal, comprising: predicting at leasta portion of a current image in a current layer based on at least aresidual coded portion of a base image in a base layer, a referenceimage, shift information for samples in the predicted current image, andoffset information indicating a position offset between at least oneboundary pixel of the reference image and at least one boundary pixel ofthe current image, the residual coded portion representing differencepixel data.
 2. The method of claim 1, wherein the reference image isbased on the base image in the base layer.
 3. The method of claim 2,wherein the reference image is at least an up-sampled residual codedportion of the base image.
 4. The method of claim 1, wherein thepredicting step comprises: upsampling at least the residual codedportion of the base image based on the shift information to obtain thereference image.
 5. The method of claim 4, wherein the shift informationis phase shift information.
 6. The method of claim 4, wherein thesamples are at least one of luma and chroma samples.
 7. The method ofclaim 4, wherein the predicting step predicts the portion of the currentimage based on at least part of the reference image and the offsetinformation.
 8. The method of claim 7, wherein the offset informationincludes left offset information indicating a position offset between atleast one left side pixel of the reference image and at least one leftside pixel of the current image.
 9. The method of claim 7, wherein theoffset information includes top offset information indicating a positionoffset between at least one top side pixel of the reference image and atleast one top side pixel of the current image.
 10. The method of claim7, wherein the offset information includes right offset informationindicating a right position offset between at least one right side pixelof the reference image and at least one right side pixel of the currentimage.
 11. The method of claim 7, wherein the offset informationincludes bottom offset information indicating a bottom position offsetbetween at least one bottom side pixel of the reference image and atleast one bottom side pixel of the current image.
 12. The method ofclaim 7, wherein the offset information includes left offset informationindicating a position offset between at least one left side pixel of thereference image and at least one left side pixel of the current image,top offset information indicating a top position offset between at leastone top side pixel of the reference image and at least one top sidepixel of the current image, right offset information indicating a rightposition offset between at least one light side pixel of the referenceimage and at least one right side pixel of the current image, and bottomoffset information indicating a bottom position offset between at leastone bottom side pixel of the reference image and at least one bottomside pixel of the current image.
 13. The method of claim 7, wherein thepredicting step obtains the offset information from a header for atleast a portion of a picture in the current layer.
 14. The method ofclaim 13, wherein the predicting step obtains the shift information froma header of a slice in the base layer.
 15. The method of claim 13,wherein the predicting step determines that the offset information ispresent based on an indicator in the header.
 16. The method of claim 13,wherein the picture is a slice.
 17. The method of claim 4, whereinpredicting step predicts the portion of the current image based on atleast part of the reference image, the offset information and dimensioninformation, the dimension information indicating at least one dimensionof the current image.
 18. The method of claim 17, wherein the dimensioninformation includes width information indicating a width of the currentimage.
 19. The method of claim 17, wherein the dimension informationincludes height information indicating a height of the current image.20. The method of clam 17, wherein the dimension information includeswidth information indicating a width of the current image and heightinformation indicating a height of the current image.
 21. The method ofclam 4, wherein the predicting step obtains the shift information from aheader of a slice in the base layer.
 22. The method of claim 21, whereinthe predicting step determines that the shift information is presentbased on an indicator in the header of the slice.
 23. The method ofclaim 4, wherein the predicting step obtains the shift information froma sequence level header in the current layer.
 24. The method of claim23, wherein the predicting step determines that the shift information ispresent based on an indicator in the sequence level header.
 25. A methodof encoding a video signal, comprising: encoding at least a portion of acurrent image in a current layer based on at least a residual codedportion of a base image in a base layer and a reference image, theresidual coded portion representing difference pixel data; and recordingshift information for samples in the predicted current image in theencoded video signal; recording offset information in the encoded videosignal, the offset information indicating a position offset between atleast one boundary pixel of the reference image and at least oneboundary pixel of the current image.
 26. An apparatus for decoding avideo signal, comprising: a decoder predicting at least a portion of acurrent image in a current layer based on at least a residual codedportion of a base image in a base layer, a reference image, shiftinformation for samples in the predicted current image, and offsetinformation indicating a position offset between at least one boundarypixel of the reference image and at least one boundary pixel of thecurrent image, the residual coded portion representing difference pixeldata.
 27. An apparatus for encoding a video signal, comprising: anencoder encoding at least a portion of a current image in a currentlayer based on at least a residual coded portion of a base image in abase layer and a reference image, recording shift information forsamples in the predicted current image in the encoded video signal, andrecording offset information in the encoded video signal, the offsetinformation indicating a position offset between at least one boundarypixel of the reference image and at least one boundary pixel of thecurrent image, the residual coded portion representing difference pixeldata.